This relates to an improved control for a paving machine and method of machine operation and, more particularly, to such a control and method for controlling paving machines having incorporate screeds that are drawn over the surface of a paved area, contouring the surface.
Paving machines of this type typically include a tractor or towing vehicle that moves ahead of a screed over a subgrade to be paved. The paving machine deposits a layer of asphalt or other paving material on the subgrade, and the thickness and contour of the asphalt layer are determined by a “floating” screed that is towed behind the towing vehicle. The screed has a plate on its lower surface that rides up over the asphalt that is deposited behind the vehicle. The screed includes a pair of forward extending tow arms that are connected to the vehicle at tow points. The tow points are raised and lowered by hydraulic cylinders on the towing vehicle. When the tow points are raised, the front edge of the screed plate is raised and the angle of attack of the screed plate is changed such that it planes upward over the asphalt that is being deposited on the subgrade just ahead of the screed. This results in the top surface of the asphalt layer rising in height and a thicker layer of asphalt on the subgrade. Conversely, when the tow points are lowered, the front edge of the screed plate is also lowered, reorienting the plate to plane downward, and lowering the top surface of the asphalt layer. It will be appreciated, that the screed smoothes the top surface of the layer of the paving material, while at the same time controlling the vertical position of this surface and the thickness of the asphalt layer.
The paving machine deposits the paving material on the subgrade so that the top surface of the paving material follows a desired elevation contour. In some instances, the top surface of the asphalt is contoured in relation to an adjacent reference surface. For example, when a second strip of asphalt is deposited on a roadbed next to a first strip of asphalt, it is desired that the surface height of the two strips match precisely at the seam where they abut. As another example, when an asphalt layer is deposited on a subgrade next to an existing street curb, it may be desired that the asphalt surface height be controlled precisely with respect to the curb. In other cases, the asphalt is contoured to match a reference set by a surveyor. For example, a surveyor may have previously surveyed a road or other surface to be paved, and set a series of stakes with a reference string line running from the top of one stake to the top of the next stake. In all of these instances, it is necessary that the vertical position of the top surface of the deposited paving material be controlled precisely with respect to a reference of some sort, and this requires that the tow points of the tow arms be controlled with precision. In other paving operations, the desired contour of the paved surface is defined in a three dimensional database, and the location of the paver, including the screed, is monitored by means of GPS receivers, laser receivers, automated total station systems, or similar systems. In these cases, the paver is operated to deposit a layer of paving material which matches in contour and thickness the parameters defined in the database.
Paving machine screeds have been controlled in a number of ways. When the level of the asphalt surface is to follow a reference surface adjacent the area to be paved, such as a previously paved surface or a string line positioned by a surveyor, it has been common to measure the vertical position of the reference surface with one or more sensors. The sensors determine the distances to the reference surface, and these distances can be used to control the height of the tow point. The tow point on the opposite side of the machine may be raised and lowered by the same amount, or it may be controlled independently using other sensors.
It will be appreciated that if the paving machine is not moving, simply raising the tow point will not cause the level of the asphalt at the trailing edge of the screed plate to change. Any change in level of the top surface of the asphalt must be accomplished slowly and without overshoot to maintain a smooth, ripple free surface on the asphalt. As a consequence, it has been common to measure the vertical reference height elsewhere along the screed. This approach may require that the level of the screed be manually adjusted from time to time, however.
The control system for a screed of this type experiences differing measurement gain, depending on where on the screed the reference height sensor is located. For example, if the height sensor location is close to the tow point of the screed tow arm, the system will be more proficient at keeping the tow point at a constant height as the paving machine moves over uneven terrain. It would be expected that this would result in a smooth asphalt surface behind the screed. However, the height accuracy, or asphalt mat thickness control at the back of the screed may not be very good, because this depends on the screed angle of attack, which is not controlled, either directly or indirectly. Since with such an arrangement, there is a significant distance between the sensor and the trailing edge of the screed, error in the angle of attack of the screed is amplified as a height error at the back edge of the screed. Conversely, if the height sensor is positioned close to the trailing edge of the screed, the screed exit height is might be thought to be closer to the target height or mat thickness, since an error in the screed angle of attack is not propagated over such a large distance. However, the feedback gain is relatively low in this case (i.e., there is only a small movement detected by the sensor in response to a relatively large change in tow point height), and hence any sensor measurement error will result in larger movements of the tow point cylinders. This, in turn, reduces the smoothness of resulting asphalt surface.
It is desirable to be able to measure the height of the asphalt at the trailing edge of the screed plate and to make an appropriate adjustment without the need for manual control by an operator. Accordingly, it is seen that there is a need for an improved paving machine control and method of machine control.